A remote control system for a car-mounted device conventionally has a remote operation function (keyless function) for locking/unlocking a door of a car by operating an operation part of a portable unit, and a smart function for locking/unlocking a door of a car and the like without operating the operation part. Without operating the operation part of the portable unit, the smart function realizes locking/unlocking of a door and the like by verifying a response signal sent back from the portable unit in response to an inquiry signal transmitted from the car.
A remote control system for a car-mounted device with the smart function in addition to the keyless function is disclosed for example in patent publication 1. Patent document 1 discloses a remote control system for a car-mounted device including a portable wireless unit with transmission means for transmitting a response signal when an inquiry signal is received, and control means for outputting a signal for unlocking a door of a car when receiving means receives a response signal transmitted after an inquiry signal transmitted at certain intervals is received, and for outputting a signal for locking a door of the car after the elapse of a predetermined period of time when a response signal is not received.
When a remote control system for a car-mounted device handles a plurality of portable units, response times of the portable units for responding to an inquiry signal from a car should be shortened. A remote control system for a car-mounted device disclosed in patent document 2 employs a spread spectrum communication system allowing multiple access in which communication time is constant irrespective of the number of portable units.
Patent Document 1: Japanese Patent Application Laid-Open No. 5-106376
Patent Document 2: Japanese Patent Publication No. 2003-500957
In the keyless function for locking/unlocking a door of a car by operating an operation part of a portable unit, transmission power should be increased for communication from a distance, and a transmission waveform with a limited transmission band should be used. When the transmission band of a transmission waveform is limited, however, high spreading gain is difficult to achieve in spread spectrum, causing a problem of degradation of signal separation performance in the smart function for responding to an inquiry signal from the car. The increase in transmission power also causes interference with another car. This necessitates the use of a different code for each car, so many codes should be used. A code sequence with a large number of codes results in degradation of separation performance. A code sequence with a large number of codes also causes degradation of signal separation performance in the smart function.